1. Field of the Invention
This invention is directed to a dentifrice composition containing acidulated fluoride compounds effective as anticaries agents and more particularly to an acidulated phosphate fluoride dual component dentifrice composition for fluoridating teeth.
2. The Prior Art
It has long been known to include fluoride containing compounds in dentifrices as anticaries agents, and it has been established that these compounds are effective to reduce the incidence of dental caries. Fluoride compounds which are deemed to be the most effective are sodium fluoride, sodium monoflurophosphate and stannous fluoride. The fluoride compounds are effective mainly due to the fluoride ions which improve the acid resistance of tooth enamel and accelerate remineralization (i.e. recalcification) of decayed teeth in their early stage when the decalcification has proceeded only slightly. The effect of improving the acid resistance of the enamel is believed to be due to the fact that the fluoride ions are incorporated into a crystal lattice of hydroxyapatite which is the main constituent of tooth enamel or, in other words, fluoride ions partially fluoridate hydroxyapatite and simultaneously repair the lattice irregularities.
The effectiveness of fluoride treatment in providing acid resistance to tooth enamel is dependent upon the amount of fluoride ion which is available for deposition on the enamel being treated. Also, using phosphate buffered NaF, the incorporation of fluoride as CaF2 into dental enamel is facilitated at lower pH levels which increase enamel solubility. Prencipe et al., Colloid and Surface Chemistry, 4th Chemical Congress of America, New York (Aug. 25-30, 1991). It is, therefore, desirable to formulate an acidulated phosphate fluoride dentifrice to enhance fluoride deposition onto and uptake into the tooth enamel.
Acidulated phosphate fluoride is disclosed in U.S. Pat. Nos. 4,080,440 and 5,603,992 as delivered in a dual component dentifrice comprised of a separately housed first acidic component containing a fluoride salt such as NaF, which with a second separately housed cationic calcium ion containing component, forms the two component dentifrice for tooth remineralization. While these two separately housed component formulations avoid any reactions between ingredients within the first and second components during storage, upon application to the teeth the fluoride salt will begin to react with the cationic calcium ion, reducing the availability of the fluoride for fluoridation of the teeth. Further, a reaction occurs during storage within the first component itself, between the acid, the sodium fluoride and the silica abrasive therein, reducing the bioavailability of the fluoride ion. The amorphous silica abrasive ingredient generally used in dentifrices is one of the most chemically inert abrasives available, however, the negatively charged oxygen atoms of the silica are protonated in the acid environment, and the resulting hydroxyl (OH) moieties hydrogen bond to the available fluoride ions released from the sodium fluoride. This fluoride-OH bond occurs in the acidulated phosphate fluoride silica abrasive systems disclosed in prior art U.S. Pat. Nos. 4,080,440, and 5,603,922 whereby, the availability of the fluoride ions present in the dentifrice to fluoridate the enamel upon application thereto is significantly reduced. The magnitude of this reduction in available fluoride is hereinafter demonstrated.
Thus, there is a clear need to formulate a stable, acidulated phosphate fluoride dentifrice, having the maximum deliverable fluoride.
In accordance with the present invention there is provided a method for the fluoridation of tooth enamel using a multicomponent dentifrice comprised of two separately housed, semi-solid aqueous components; the first component containing sodium fluoride as a source of fluoride ions, in an orally acceptable vehicle having a pH of at least about 7.5 and, the second component which is fluoride free containing both a source of phosphate ions and an acid to provide a pH of from about 2.5 to about 5.0, in an orally acceptable vehicle; each component containing a siliceous abrasive and each component being free of any cationic calcium ion containing component, such as water soluble calcium salts, whereby upon mixing of the components, a mixture having a pH of from about 4.0 to about 6.0 is formed, whereby upon application of the mixture to the teeth, the availability of fluoride ions is enhanced.
There has been no recognition in the prior art of the significant loss of fluoride availability during extended periods of storage, resulting from the interaction of generally inert silica abrasives, with sodium fluoride, when NaF is present as the fluoride source in dentifrices having an acid environment. In accordance with the present invention, as will be further demonstrated herein, the enhanced fluoride availability resulting from separating the acid and the fluoride in separate dentifrice components, each containing a silica abrasive, is unexpected and significant.